1. Field of the Invention
The invention relates to a receiving device that receives a signal and more particularly to a receiving device that demodulates using frequency modulation (FM) demodulation, smoothes the demodulated signal to generate a reference voltage, compares the reference voltage with the demodulated signal, and generates a binary signal showing received data.
2. Description of the Related Art
A control system is conventionally known that can remotely control a control object unit by transmitting control data from a mobile device to the control object unit by radio. Specifically, for example, a keyless entry system for an automobile is known that uses a transmitter-receiver employing an FSK modulation method for transmission and reception of the control data.
The transmitter-receiver using the FSK modulation method transmits and receives an FM modulated wave generated by frequency-modulating a carrier wave with the control data, which is a binary signal, to be transmitted and received.
The receiving device frequency-converts the received signal into an intermediate frequency signal and FM-demodulates the intermediate frequency signal using an FM detector to generate a demodulated signal amounting to a voltage signal corresponding to the frequency of the received signal. The receiving device further waveform-shapes the demodulated signal to a binary signal in order to recover the binary signal corresponding to the control data used by a transmitter to frequency-modulate a carrier wave. Descriptions of such a receiver can be found, for example, in JP 2005-236556A and JP 2006-203614A.
In the exemplary receiving device a waveform-shaping circuit for binarizing the demodulated signal is constructed to compare the demodulated signal with a reference voltage to binarize the demodulated signal. However, when the reference voltage is kept constant, the control data may not be normally recovered due to influences such as frequency variations of the carrier wave generated at the transmitter side and of a local oscillating signal used for frequency conversion of the received signal into the intermediate frequency signal at the receiving device side or noise components superimposed in the demodulated signal.
As shown in FIG. 9A, a waveform-shaping circuit is known and is configured in such a manner that a demodulated signal is smoothed through a smoothing circuit 84 formed of a resistance R1 and a capacitor C1 to generate a reference voltage and the reference voltage and the demodulated signal are compared through a comparator 86 to generate a binary signal.
It should be noted that FIG. 9A is a schematic construction a circuit of the conventional receiving device constructed in such a manner that a low pass filter (LPF) 82 is arranged downstream of a FM detector 80 for FM-demodulating an intermediate frequency (IF) signal to eliminate high-frequency noise components from the demodulated signal. After noise elimination, the demodulated signal is separated into two lines, one of which is inputted into a converter 86 through an input resistance R10 and the other of which is inputted into the converter 86 through the smoothing circuit 84 formed of the resistance R1 and the capacitor C1.
In a case where the demodulated signal is smoothed to generate the reference voltage as described above, increasing the time constant of the smoothing circuit 84 causes ripples of the reference voltage to be made small, stably carrying out the binarization of the demodulated signal.
When the time constant of the smoothing circuit 84 is made large for stably carrying out the binarization of the demodulated signal however, time, from a point of starting reception of a transmitted electric wave from the transmitter to a point where the reference voltage reaches a voltage value corresponding to the demodulated signal, is increased. The response time until the data can be normally recovered is made longer.
As shown in FIG. 9B, since the FM detector 80 outputs a demodulated signal a voltage value of which changes in accordance with a frequency of a received signal, such as the IF signal, when a jamming signal exists within a frequency zone, such as the IF zone, in which the FM detector 80 can carry out the FM demodulation, a voltage value of the demodulated signal outputted from the FM detector 80 temporarily increases or decreases as compared to the demodulated signal only at the time of demodulating an object signal, which is contained in the received signal along with the jamming signal, if present.
When the demodulated voltage temporarily increases or decreases due to jamming, the reference voltage also varies likewise. Therefore, even if the jamming signal stops, rapid resumption of the data reception can not occur.
Specifically, when a frequency of the jamming signal is lower than that of the object signal, the demodulated signal is lower than a normal voltage value corresponding to the object signal and the reference voltage also decreases accordingly. In such a case, as shown in FIG. 9C, even if the jamming signal stops and the demodulated signal recovers back to a voltage value in accordance with the object signal, the reference voltage gradually increases with the time constant of the smoothing circuit 84. Therefore, even if a state of the demodulated signal varies to from a high level to a low level, or from a low level to a high level, in response to the received data, the output from the comparator 86 is fixed to a high level until the reference voltage increases to a voltage variation region of the demodulated signal.
More specifically, when the frequency of the jamming signal is higher than the object signal, the demodulated signal is higher than the normal voltage value corresponding to the object signal and the reference voltage also increases accordingly. In such a case, as shown in FIG. 9B, even if the jamming signal disappears and the demodulated signal recovers back to the normal voltage value in accordance with the object signal, the reference voltage gradually decreases with the time constant of the smoothing circuit 84. Therefore, even if the demodulated signal varies to a state from a high level to a low level or from a low level to a high level response to the received data, the output from the comparator 86 is fixed to a low level in the duration until the reference voltage decreases to the voltage variation region of the demodulated signal.
When the receiving device does not receive the transmitted electric wave from the transmitter, the output level from the FM detector 80 becomes low. Thereafter, when the reception of the transmitted electric wave from the transmitting device starts, as in the case where the jamming signal of the low frequency wave stops as shown in FIG. 9C, the output from the comparator 86 is fixed to a high level until the reference voltage increases to a voltage variation region of the demodulated signal. In consequence, the response time during which the data can be normally recovered is made longer.
On the other hand, such a problem is solved by making the time constant of the smoothing circuit 84 be small. However, when the time constant of the smoothing circuit 84 is made small, the voltage variation (ripple) of the reference voltage is made large. Therefore, the circuit is easily influenced by the noise superimposed in the demodulated signal and the binary signal (data) can not be recovered with high accuracy.